The present invention relates to a transmitter and, more particularly, to a portable, battery-powered transmitter including a current control circuit for controlling the output power of the transmitter over a wide range of variations in battery voltage, rf input power and frequency, and ambient temperature.
There are many applications in which it is desirable for a user of a portable, battery-operated, low-power transmitter to initiate a rf transmission, for example, in the form of a rf signal, by which some action can be controlled. In such applications, it is generally a requirement that the output of the transmitter be established and maintained at a sufficiently high, detectable level over a wide range of variations in battery voltage, rf input power and frequency, and ambient temperature. This control of the output power is especially important in the case of the reduction or decline over a period of time of the value of voltage of the battery due to normal discharge or current drain. In this situation, it is desirable that the discharge or drain of the battery be minimized as much as possible, and over as long a period of time as possible, so as to guarantee or insure a sufficiently long battery mission life.
Apparatus which has been proposed heretofore for achieving the above objectives and requirements includes reflectometers and peak rf detectors. In the case of a reflectometer, this device is coupled by suitable circuitry to the output of the transmitter and periodically samples the level of the output power of the transmitter. The sampled power values are converted to voltages and compared in value with a reference voltage. The results of the comparison operations are used to maintain the output power of the transmitter at a reasonable, constant value. This control of the output power of the transmitter is achieved irrespective of the values of battery voltage or current. The abovedescribed reflectometer, while providing precise control, is a passive device requiring considerable and costly circuitry, including coils, strip line assemblies, capacitors, etc., and further requires a precise voltage reference and complex temperature compensation and calibration. In addition, the reflectometer, by virtue of being coupled to the output circuit of the transmitter, adds insertions loss to the circuit thereby dissipating the output power of the transmitter and placing an additional power gain burden on the transmitter.
A peak detector as mentioned hereinabove, when utilized to control the output power of a transmitter, is coupled to the output circuit of the transmitter and samples peak power. However, this type of detector is useful only for narrowband applications because of possible impedance variations, for example, in the case of unexpected antenna-related malfunctions (e.g., high VSWR conditions). In addition, this type of detector, like the aforedescribed reflectometer, requires coupling circuitry for coupling the device to the output circuit of the transmitter, thereby adding to the overall cost of the transmitter.